FIP4 C3-3 is a hybridoma which secretes both IgM and IgG3 anti- SSSIII antibodies simultaneously. Immunofluorescent studies indicate that most individual cells are "double secretors." The line has been cloned repeatedly, and is a proved double-secretor because: (1) individual cells stain both for IgM and IgG3; (2) the line incorporates 14 C lysine into both IgM and IgG3 in vitro; (3) supernates from individual clones in microwells contains both IgM and IgG3 antibodies to SSSIII; (4) the line produces IgM and IgG3 paraproteins when transplanted into mice. It was found in the cloning studies that single IgM or IgG3 producers were fairly common, about 1 in 20. Molecular analysis with probes for IgM and IgG3 has been begun. Southern blot analysis of double producing and of IgM producing lines shows no DNA rearrangement thus far. One IgG3 single producer shows an IgG3 rearrangement.